Integrate Combine Into an App

Aug 5 2021 · Swift 5.4, macOS 11.3, Xcode 12.5

Part 1: Define a View Model

04. Use Publishers in the ViewModel

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Previous episode: 03. Create Data Publishers Next episode: 05. Use @ObservedObject to Monitor State

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In the last episode, we made our own custom publishers that, in the end, started with a publisher (in this case a dataTaskPublisher from URLSession), and we performed some operations on that publisher, the last of which was erasing the type signature to AnyPublisher. This takes advantage of the fact that Operators are both Publishers and Subscribers.

private let jokesService: JokeServiceDataPublisher
private let translationService: TranslationServiceDataPublisher
public init(jokesService: JokeServiceDataPublisher = JokesService(),
            translationService: TranslationServiceDataPublisher = TranslationService()) {
  self.jokesService = jokesService
  self.translationService = translationService
  .map { _ in false }
  .assign(to: \.fetching, on: self)
  .store(in: &subscriptions)
public func fetchJoke() {
  // 1
  fetching = true
  // 2
  jokeSubscriptions = []
    .decode(type: Joke.self, decoder: Self.decoder)
    .replaceError(with: Joke.error)
    .receive(on: DispatchQueue.main)
    .handleEvents(receiveOutput: { [unowned self] in
      self.joke = $0
    .filter { $0 != Joke.error }
    .flatMap { [unowned self] joke in
      self.fetchTranslation(for: joke, to: "es")
    .receive(on: DispatchQueue.main)
    .assign(to: \.joke, on: self)
    .store(in: &jokeSubscriptions)
func fetchTranslation(for joke: Joke, to languageCode: String)
  -> AnyPublisher<Joke, Never> {

  guard joke.languageCode != languageCode else {
    return Just(joke).eraseToAnyPublisher()
  return translationService.publisher(for: joke, to: languageCode)
    .decode(type: TranslationResponse.self, decoder: Self.decoder)
    .compactMap { $0.translations.first }
    .map {
           value: joke.value,
           categories: joke.categories,
           languageCode: languageCode,
           translationLanguageCode: languageCode,
           translatedValue: $0)
    // 5
    .replaceError(with: Joke.error)